1. Field of the Invention
The present invention concerns the wire electrodes used in the machining of metal parts by the spark erosion process.
2. Description of the Prior Art
In such machining, as described in document FR-A-2 418 699, for example, a wire electrode is driven in longitudinal translation and a portion of said wire is guided and stretched along a straight line segment that is displaced laterally along a path near a metal part to be machined. An electrical generator produces a potential difference between the part to be machined and the metal wire forming the electrode. Machining occurs in the machining area between the electrode wire and the metal part and progressively erodes the part and the wire.
It has been a long-standing aim to improve the qualities of spark erosion electrode wire by combining good mechanical tensile strength, good electrical conductivity of the wire and more regular production of erosive sparks in the machining area between the electrode wire and the part to be machined.
For example, document U.S. Pat. No. 4,287,404 describes a method and a device for manufacturing a spark erosion electrode wire having a filamentary core surrounded by a layer of metal having a low boiling point, such as zinc, cadmium, tin, lead, antimony or bismuth. The metallic surface layer is deposited by an electrolytic deposition step followed by a wire drawing step. A method of this kind has the drawback of producing an electrode wire with a surface layer that vaporizes too quickly and provides insufficient protection of the core during spark erosion.
It has been found advantageous to heat the wire after electrolytic deposition of the surface layer of metal having a low boiling point, as taught in document EP-A-185 492, in order to produce a diffused alloy. However, a method of this kind cannot produce at high speed an electrode wire having a thick surface layer of diffused alloy.
Document U.S. Pat. No. 4,169,426 describes another method of manufacturing a spark erosion electrode wire having a filamentary core surrounded by a metal layer in which an input conductive wire is passed continuously through a bath of molten metal, after which the wire is rapidly cooled to avoid the formation of intermetallic compounds at the interface between the filamentary core and the metallic surface layer. The wire is then heat treated at 320.degree. C. for several minutes before it is drawn down to the required final diameter. A method of this kind also has the drawback that it is slow, in particular because of the heat treatment time needed to enable subsequent wire drawing without damaging the metallic surface layer.
Document CH-A-655 265 proposes to preheat the input conductive wire by the Joule effect, by passing an appropriate electric current through it, before it is passed through a bath of molten metal. The aim is to increase the speed of manufacture of the spark erosion wire. On leaving the bath of molten metal, the wire is cooled rapidly to prevent the formation of intermetallic compounds at the interface between the core and the coating. The wire can then be drawn down to the required final diameter. This document clearly teaches that the wire drawing is possible because no intermetallic compounds are formed at the interface between the core and the coating.
However, it is very difficult to carry out wire drawing afterwards without seriously damaging the metallic surface layer. This layer tends to become detached from the core because of the mechanical stresses applied by the wire drawing die.
Documents U.S. Pat. No. 3,391,450 and FR-A-1 526 442 describe a combined method of annealing and tinning a copper wire in which the annealing is done by passing an electric current through the wire before, during and after its passage through a bath of molten tin at a relatively low temperature, the wire being heated by the current to a temperature close to that of the bath of molten tin. There is no mention of wire drawing, of applications to spark erosion or of problems with adhesion of the metallic surface layer.
The problem addressed by the present invention is therefore that of designing simple and inexpensive means for improving the adhesion of the metallic surface layer to the filamentary core in a fast method of manufacturing spark erosion electrode wire passing continuously through a bath of molten metal in order to enable subsequent drawing of the wire without significant damage to the metallic surface layer. The aim is thus to produce at high speed a spark erosion electrode wire having a thick surface layer of diffused alloy.
The invention stems from the surprising observation that the adhesion of the metallic layer to the central core is considerably improved if the wire is heated sufficiently by the Joule effect during its passage through the bath of molten metal.
This goes directly against the teaching of documents U.S. Pat. No. 4,169,426 and CH-A-655 265 which recommend minimal heating of the filamentary core to prevent the formation of intermetallic compounds at the interface between the core and the metallic surface layer.